Implement Merkle tree serialization to protobuf

[artemananiev]

One of the aspects of public merkle tree APIs feature is that all platform inputs and outputs should be in protobuf. In particular, platform state is saved to disk every 15 minutes, and these snapshots are currently just binary files with no clear schema. These files should be changed to protobuf.

#13764 determines if this work will be to flush the virtual node cache on every snapshot, or to serialize it as protobuf.

[poulok]

After Virtual Mega Map, is the only part left of this the Virtual Node Cache?